The most pertinent prior art the applicant and his attorney are aware of is that which is to be described hereinbelow.
My associates have been making duo density rotors of silicon nitride in the following manner. A rotor blade ring is formed, for example, in a slip casting operation. This blade ring has an annular ring portion with a plurality of complex shaped blade surfaces projecting outwardly therefrom. The blade ring is formed by slip casting silicon particles into a mold suitable for forming the complex shape desired. The vehicle of the slip is withdrawn through surfaces of the mold as is well known in the art to leave behind a consolidated mass of silicon particles in the shape of a blade ring. This blade ring is subsequently subjected to a nitriding operation in which the silicon particles are converted to silicon nitride. All but an interior annular surface of the silicon nitride article is subsequently encapsulated with additional material which is transformed into silicon nitride to form a ring shaped unit. The article is encapsulated so that it can withstand the temperatures and pressures of a hot press bonding operation in which a central hub is simultaneously formed and bonded to an interior annular surface of the blade ring.
The general process previously used for carrying out this operation is disclosed in U.S. patent application Ser. No. 571,897, filed Apr. 25, 1975, for a Method of Forming a Duo Density Silicon Nitride Article, which application is now abandoned. This application was assigned to the same assignee as this application and is hereby incorporated by reference.
The cited application teaches that the blade ring should be treated with a release material prior to having the same encapsulated. The purpose of the release material is to permit easy removal of the encapsulating material after the hot press bonding operation. The application cited teaches that a uniform thin layer of a release material such as boron nitride should be applied to all surfaces of the blade ring which are to be encapsulated.
I have found that the placement of a uniform coating of a release material is not the best procedure for obtaining a finished article. When I follow the procedure described in the above noted patent application, I often obtain finished articles in which cracks were found in the blades and ring portion thereof.
In conducting further work in order to develop a process for producing a crack-free article, I discovered that a relatively thin layer of release material should be applied to those surfaces of the ceramic body which are to be a load reaction surface of the body during a hot pressing operation. By a load reaction surface, I mean a surface upon which reaction forces will be developed because of the direct application of a force on another surface during the hot pressing operation. All other surfaces should have a relatively thick coating of the release material thereon.